1. Field of the Invention
The present invention concerns an x-ray computed tomography apparatus of the type having at least one anode ring in a vacuum housing surrounding an examination volume, wherein a focus of an x-ray source revolves on the anode ring to irradiate the examination volume with an x-ray beam from different directions, a detector system arranged on a rotating frame that can rotate on a system axis to receive the x-ray radiation escaping from the examination volume, wherein the detector system and the focus can be rotated synchronously around the system axis with a rotation angle offset by 180°, and a computer to process the measurement values acquired by the detector system.
2. Description of the Prior Art
X-ray computed tomography systems are used in medical imaging (for example) in order to acquire images of the inside of the body of a patient. An x-ray computed tomography apparatus includes, among other things, a device to generate x-ray radiation, an x-ray detector, and a patient positioning table with which the examination subject can be moved through the examination volume along a system axis (known as the Z-axis) during the examination. The device for generation of x-ray radiation generates an x-ray beam that emanates from an x-ray focus rotating around the examination volume. In examinations the x-ray beam, which expands in a fan shape in a slice plane of the examination volume (X-Y plane) that is perpendicular to the system axis, penetrates a slice of the examination subject (for example a body slice of a patient) and strikes the detector elements of the x-ray detector situated opposite the x-ray focus. The angle at which the x-ray beam penetrates the body slice of the subject, and possibly the position of the patient positioning table, normally vary continuously during the image acquisition with the computed tomography apparatus.
The intensity of the x-rays of the x-ray beam which strike the x-ray detectors after penetrating the patient is dependent on the attenuation of the x-rays by the patient. Every detector element of a detector row of the x-ray detector thereby generates a voltage signal depending on the intensity of the received x-ray radiation, which voltage signal corresponds to a measurement of the global transparency of the body for x-rays from the x-ray tube to the corresponding detector element. A set of voltage signals of the detector elements of a detector row which correspond to attenuation data and were acquired for a specific position of the x-ray source relative to the patient is designated as a projection. A set of projections which were acquired at various positions during the movement of the x-ray focus around the patient is designated as a scan. The x-ray computed tomography apparatus acquires many projections from different positions of the x-ray focus relative to the body of the patient in order to reconstruct an image which corresponds to a two-dimensional slice image of the body of the patient or a three-dimensional image. A volume scan that includes multiple rotations of the x-ray focus around the examination volume with a feed movement of the patient table in the Z-direction is implemented to acquire multiple slice images or a three-dimensional image. The established method for reconstruction of a slice image or three-dimensional image from acquired attenuation data is known as the filtered back projection technique. The image reconstruction is normally implemented with an image computer that receives the measurement data from the detector elements and processes it further.
In x-ray computed tomography apparatuses the third generation, the rotating x-ray focus is generated by an x-ray tube that, like the x-ray detector, is attached to a rotating frame (gantry) that can rotate around the examination volume. The rotation speed of the rotating frame was steadily increased in recent years in order to achieve faster scan speeds in the image acquisition. However, for mechanical stability and safety a limit has been reached for computed tomography apparatuses of the third generation because a distinct increase of the rotation speed of the rotating frame is no longer allowed due to the moving masses and the high acceleration forces resulting from this.
Conventional rotating piston or rotating anode x-ray tubes are most commonly used as x-ray tubes in computer tomographs of the third generation. X-ray tubes of this design are characterized by the anode being fashioned at least in the shape of a ring and can be rotated around a rotation axis, wherein the electron beam coming from the cathode strikes the anode at a focal spot that is stationary relative to the rotation axis. The focal spot thus describes an annular focal path on the anode due to the rotation of the anode. As a result of the rotation of the anode, the heat arising at the focal spot upon the electron beam striking the anode is distributed in the anode, which contributes to an increase of the anode service life and in particular increases the beam intensity of the x-ray radiation generated at the anode. It thereby applies that: the higher the path speed of the focal spot on the anode, the greater the power density of the x-ray radiation that is generated. The power density of conventional rotating piston or rotating anode tubes can thus be increased via an increased anode rotation frequency or a greater anode diameter. Corresponding path speeds of over 100 m/s are known in the prior art.
The x-ray power emitted by conventional rotating piston or rotating anode x-ray tubes is consequently independent of the rotation speed of the gantry. For example, if the gantry rotation speed is reduced, the x-ray dose acting on the examination volume is increased without the x-ray tube thereby being overloaded.
An x-ray computed tomography apparatus is known from EP 0 377 070 A1 in which the radiation detector is fashioned as in an x-ray computer tomograph of the third generation; however, the x-ray tube possesses a stationary annular anode entirely enclosing the examination region. The design of the x-ray source as a closed anode ring enables a good heat dissipation or distribution in the anode, an increased electron stream from cathode to anode, and thus an increased x-ray power. The cathode and the radiation detector situated 180° opposite to it are connected with one another and rotate in the same direction of rotation and synchronously around a common rotation axis that corresponds to the z- or system axis. A disadvantage in the x-ray tube described in EP 0 377 070 A1 is that the path speed is limited, and therefore the power that can be achieved with the x-ray tube at relatively low tube power is limited. Furthermore, an increase of the x-ray dose via a slower gantry rotation as with computer tomographs of the third generation is not possible. Rather, this would lead to a more severe stressing (loading) of the anode. The x-ray tube power is additionally limited in scans with a stationary gantry, for example in topogram acquisitions.